Recently, radio frequency identification (RFID) technology has gained tremendous popularity as a device for storing and transmitting information. RFID technology utilizes a tag transponder, which is placed on an object, and a reader, also referred to herein as an interrogator, to read and identify the tag. RFID technologies are broadly categorized as using either “active” tags or “passive” tags. Active tags have a local power source (such as a battery) so that the active tag sends a signal to be read by the interrogator. Active tags have a longer signal range. Passive tags, in contrast, have no internal power source. Instead, passive tags derive power from the reader, and the passive tag re-transmits or transponds information upon receiving the signal from the reader. Passive tags have a much shorter signal range (typically less than 20 feet).
Both categories of tags have an electronic circuit that is typically in the form of an integrated circuit or silicon chip. The circuit stores and communicates identification data to the reader. In addition to the chip, the tag includes some form of antenna that is electrically connected to the chip. Active tags incorporate an antenna that communicates with the reader from the tag's own power source. For passive tags, the antenna acts as a transducer to convert radio frequency (RF) energy originating from the reader to electrical power. The chip then becomes energized and performs the communication function with the reader.
On the other hand, a chipless RFID tag has neither an integrated circuit nor discrete electronic components, such as the transistor or coil. This feature allows chipless RFID tags to be printed directly onto a substrate at lower costs than traditional RFID tags.
As a practical matter, RFID technology uses radio frequencies that have much better penetration characteristics to material than do optical signals, and will work under more hostile environmental conditions than bar code labels. Therefore, the RFID tags may be read through paint, water, dirt, dust, human bodies, concrete, or through the tagged item itself. RFID tags may be used in managing inventory, automatic identification of cars on toll roads, security systems, electronic access cards, keyless entry and the like.
Antennas are an element of RFID tags that are typically prepared via stamping/etching techniques, wherein a foil master is carved away to create the final structure. Additionally, the antenna may be formed via a lithographic press. See Travis et al., U.S. Patent Publication No. 2006/0260493.
The RFID antenna may also be printed directly on the substrate using a conductive metal ink, The ink is printed on a substrate, followed by high temperature sintering in order to anneal the particles and to create a conductive line on the substrate. Precisia, L.L.C., for example, produces both solvent- and water- based conductive inks that are formulated specifically for lithographic, gravure, rotary screen and flexographic printing.
DuPont's 5033 Conductive Ink is a conductive thick film paste, that allows for the screenprinting of an antenna onto a substrate.
Dow Coming's Highly Conductive Ink is also suitable for screenprinting. However, the inks require an additional curing step.
Paralec offers inks, pastes and toners under the trademark PARMODO®, that is printed onto a substrate, using conventional printing processes. After printing, the substrate must be cured to provide pure metallic conductors. However, the PARMOD® pastes are very viscous materials that are not suitable for digital printing applications such as inkjet printing. Similarly, a low-cost metal antenna comes from the collaboration between the England-based QinetiQ and Coates Screen. With the QinetiQ method of “growing” antennas, specially formulated ink is printed on a substrate material, such as cardboard or polystyrene. The substrate is then passed through an electroless solution, which uses chemicals, to deposit metal onto a surface. The metal in the solution reacts with chemicals in the ink, and forms deposits on the substrate where there is ink.
CARCLO® also offers conductive inkjet technology. However, the ink requires a UV curable component, a catalyst, and a wet eletroless plating step with corrosive metal baths
HANITA COATINGS® develops and manufactures pure copper antennas and print substrates optimized for printing by conductive inks. Hanita's RFID antennas consist of heat stabilized PET films with a conductive ink receptive coating specifically to improve the adhesion of the conductive ink to the substrate. However this approach requires a multi-step process that involves vapor deposition of a base metal layer, followed by an insulation layer, and a wet electroplating step. This process is not amenable for direct printing on paper substrates in a single pass.
Alternatively, metal fibers may be incorporated directly into the substrate. For example, one chipless RFID technology from INKODE® uses embedded aluminum fibers that are embedded into paper and act as radar antenna. When subjected to radar waves (24 GHz super high frequency (SHF)), the fibers act as resonators, and backscattering of the waves occurs. A detector receives each of these backscattered waves, with the intensity being dependent on the volume, length and orientation of the fibers. Given a collection of such fibers in a random array, a “signature” pattern of backscatter is produced. This signature pattern can be converted with the appropriate algorithm to create a unique binary code that makes each tag unique. The INKODE® concept has several limitations. One drawback is that the fibers are embedded into the paper, and thus must be incorporated during the papermaking process as a furnish additive. Another drawback is that because aluminum fibers that are cut to the appropriate ¼ wavelength is required, the process is costly and tedious.
As a suitable metal material to be used in the ink, although particulate metal materials may be used in the ink preparation, the superior characteristics of metallic nanoparticle materials in ink applications yields a better product. Metallic nanoparticles are particles having a diameter in the submicron size range. Metallic nanoparticles have unique properties, which differ from those of bulk and atomic species. Metallic nanoparticles are characterized by enhanced reactivity of the surface atoms, high electric conductivity and unique optical properties. For example, metallic nanoparticles have both a lower melting point and a lower sintering temperature than that of bulk metal. In particular, this sharply reduced melting temperature of metallic nanoparticles make them especially suited for conductive ink applications.
Metallic nanoparticles are either crystalline or amorphous materials. They can be composed of pure metal, such as silver, gold, copper, aluminum, etc., or a mixture of metals, such as alloys, or core of one or more metals, such as copper covered by a shell of one or more other metals such as gold or silver. The nozzles in an inkjet printing head are approximately 1 μm in diameter. In order to jet a stream of particles through a nozzle, the size of a particle should be less than approximately one-tenth of the nozzle diameter. This means that in order to inkjet a particle, its diameter must be less than about 100 nm.
Nickel has been used for conductive inks for a very limited extent because of its relatively low conductivity (approximately 4 times less than that of copper or silver). Gold and silver can provide good conductivity, but are relatively expensive. Moreover, gold and silver require high temperatures for annealing, which can pose a challenge for printing on paper and plastic substrates. Copper provides good conductivity at a low price (about one percent of that of silver). Unfortunately, copper is easily oxidized and the oxide is non-conductive. Conventional copper-based nanoparticle inks are unstable and require an inert/reducing atmosphere during preparation and annealing in order to prevent spontaneous oxidation to non-conductive CuO or Cu2O. Copper polymer thick film (PFT) inks have been available for many years and can be used for special purposes, for example, where solderability is required. Another interesting strategy is to combine the advantages of both silver and copper. Silver plated copper particles are commercially available, and are used in some commercially available inks. Silver plating provides the advantages of silver for inter-particle contacts, while using the cheaper conductive metal (copper) for the bulk of the particle material. Thus, the only reliable means of preparing copper antennas is via electroplating on an existing metal surface.
There exists a need for a more simplified and cost-effective method of printing conductive metal markings, in the form of an inkjet ink, directly onto a substrate under ambient conditions.